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Author Topic: Too much power, too many subs?  (Read 2145 times)

Leonardo Wood

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Too much power, too many subs?
« on: March 31, 2014, 12:52:19 am »

Greetings to all! I've never posted here before, but I have a sound problem that needs solving and didn't even consider posting elsewhere. I knew I'd get good responses here!

If there's such a thing as a good problem, then that's what I've got. I have an excessive amount of low-end capability---that is, relative to the FOH volume levels that we typically run at the church where I oversee sound. In short, we have a pair of JBL PRX615M powered speakers for FOH and two JBL STX828S subs. One sub is powered by a Crown XTi4002 (3,200 watts bridged) and the other by a Crest Pro8200 (4,500 watts bridged).  For the time being, we set up each week in a school auditorium that seats around 800 or 900, and the acoustics are decent, but not great. 

I'd like to be able to feel the bass (up front near the subs, not in the back...we're not aiming to be that loud).  The only two sources that really drive the subs are kick drum and bass guitar.  I DO NOT want either the kick or electric bass to overtake the mix at the expense of everything else, since I'm looking for balanced sound. I've considered acquiring 1/3 octave EQ's or perhaps parametric EQ's so that I can boost a narrow band of specific low frequencies for the kick and bass, with the hope of creating a palpable low frequency thump while keeping the overall (or apparent) volume of the overall mix at reasonable levels.

Am I on the right track, or is there a better way to put this subwoofer power to good use? Or, do I just have a lot more bass capacity than I can use?

Thanks in advance for your suggestions.

Leonardo
« Last Edit: March 31, 2014, 01:37:32 am by Leonardo Wood »
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Nils Erickson

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Re: Too much power, too many subs?
« Reply #1 on: March 31, 2014, 04:49:08 am »

Leonardo,

One option is to try feeding the subs from a post fader aux send.  Mix your mains as you would normally; then, you can add whatever channels to the subs while keeping everything else out (by using the aux send).  You can add thump to taste this way.  This may fix your problem of them being out of balance with the mains, as you'll have separate control. 

When you do this, pay careful attention to how things sound around your crossover point; you may find yourself needing to do some cuts around here if you are adding lots of sub, as you can somewhat easily change the acoustic crossover point...

Anyway, there are a lot of solutions to your problem, this is just one that I like...  I'm sure others will chime in with a pile of other opinions.  Certainly it is worth trying to see if you like it.

Cheers,
Nils
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Mark Rombouts

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Re: Too much power, too many subs?
« Reply #2 on: March 31, 2014, 08:24:52 am »

By using these two amplifiers you are getting different output from both speakers.

XTI has a DSP unit in it, and will have some latency. Crest doesn't have this. Have you ever tried running both subs form one of these amps in stereo mode ? You will loose som power of course, but it may be much beter sounding.
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TJ (Tom) Cornish

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Re: Too much power, too many subs?
« Reply #3 on: March 31, 2014, 10:16:52 am »

Greetings to all! I've never posted here before, but I have a sound problem that needs solving and didn't even consider posting elsewhere. I knew I'd get good responses here!

If there's such a thing as a good problem, then that's what I've got. I have an excessive amount of low-end capability---that is, relative to the FOH volume levels that we typically run at the church where I oversee sound. In short, we have a pair of JBL PRX615M powered speakers for FOH and two JBL STX828S subs. One sub is powered by a Crown XTi4002 (3,200 watts bridged) and the other by a Crest Pro8200 (4,500 watts bridged).  For the time being, we set up each week in a school auditorium that seats around 800 or 900, and the acoustics are decent, but not great. 

I'd like to be able to feel the bass (up front near the subs, not in the back...we're not aiming to be that loud).  The only two sources that really drive the subs are kick drum and bass guitar.  I DO NOT want either the kick or electric bass to overtake the mix at the expense of everything else, since I'm looking for balanced sound. I've considered acquiring 1/3 octave EQ's or perhaps parametric EQ's so that I can boost a narrow band of specific low frequencies for the kick and bass, with the hope of creating a palpable low frequency thump while keeping the overall (or apparent) volume of the overall mix at reasonable levels.

Am I on the right track, or is there a better way to put this subwoofer power to good use? Or, do I just have a lot more bass capacity than I can use?

Thanks in advance for your suggestions.

Leonardo
Welcome to PSW.

It's unclear to me what your problem is - what is happening now that is undesirable?  Too many subs and/or too much power can be solved easily by moving the fader down slightly.

Reading between the lines, it seems that you are having trouble with low-frequency definition.  There could be several components to this:
- Channel EQ - you may need to learn how to adjust the board channel EQ for more punch.  A parametric EQ with variable frequency and Q is very helpful here.
- As mentioned, using aux or group-fed subs can help by only sending the items to the subs that need to be in the subs
- System alignment and/or placement.  As Mark mentioned, you're running different gear on different sides. I agree that one amp with one sub per channel is probably better for you guys.

Graphic EQs are fairly useless.  System issues should be solved in some kind of DSP with parametric EQ and delay alignment.  Channel EQs should also be parametric for the most control.  You may be wise to hire a local professional to come and help you do a system setup and run sound for one Sunday service.  A small investment in some help from a pro will likely teach you a lot.

If you post your location, there may be someone here local to you that could help.
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Leonardo Wood

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Re: Too much power, too many subs?
« Reply #4 on: March 31, 2014, 10:50:12 am »

By using these two amplifiers you are getting different output from both speakers.

XTI has a DSP unit in it, and will have some latency. Crest doesn't have this. Have you ever tried running both subs form one of these amps in stereo mode ? You will loose som power of course, but it may be much beter sounding.


Actually, no, I haven't tried running both subs on the Crest in stereo mode. In fact, I haven't even run one on the Crest! Let me explain...

We started out with just the Crown amp and one STX828, in addition to using a PRX618S (single 18" driver, self powered) that we had from an earlier purchase. Via an Architectural Acoustics signal processor, I sent one feed to the little sub at 0dB and a separate signal to the STX828 at -6dB, since this sub combination was so lopsided power-wise. I soon realized this was a really bad mismatch in terms of power and quality, and also for various acoustical reasons. (The PRX618 is not terrible on its own, but it's quite inferior to the STX828 when compared side by side.) 

While waiting for the second STX828 to arrive---the one that has not even been used yet---I tried just the existing STX828 without the PRX sub, and I increased the feed from -6dB to 0dB. That's when I realized I was going to have a challenge harnessing the power of two of these things! With just the one and the 6dB input increase, the LF output was massive relative to everything else.

Standing close to the sub operating at that level, I'm sure a listener could feel the bass. But to make the overall mix useable, I'd have to bring up the top end by quite a bit, and that would be too loud for our purposes, which brings me back to my original dilemma.

Perhaps by selectively boosting some very low frequency from the kick or bass guitar (probably kick, I would think), I'd be able to get a powerful thump from the subs but without having to raise the higher frequencies too much. I've also thought about a sub-harmonic synthesizer. I've never used one, and I'm doubtful that such a thing is going to help. Another thing I've considered doing is using the Architectural Acoustics processor to cut everything below 50Hz. Our music doesn't have any significant content below that frequency anyway, and I believe that signals below 50Hz from the kick and bass guitar just add to the boominess. But there goes my idea of boosting very low frequencies!
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Robert Weston

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Re: Too much power, too many subs?
« Reply #5 on: March 31, 2014, 10:59:53 am »

Going out on a limb... Sounds like this is an issue of improper (or incorrect) mixing technique.  It's true you can have too much power, but I think that's only if you don't have the mix setup properly (either gain levels are too high, channel EQ not set appropriately, FOH EQ being used, etc...)

Also - depending on the size of the room and where you are standing when testing, you could be feeling a standing wave; which can skew your observations.

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Duane Rodakowski

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Re: Too much power, too many subs?
« Reply #6 on: March 31, 2014, 11:18:37 am »

+1 What Mark Rombouts said.

Can't say this is the only problem, it sure seems like contributing factor.

Secondly, driving subs from an aux bus is an excellent way to control your low end, try that too.
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Brad Weber

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Re: Too much power, too many subs?
« Reply #7 on: March 31, 2014, 11:22:40 am »

What settings are you using for the crossover between the PRX615M mains and the subwoofers?  Have you gone through the system gain structure?  Where are the front panel input attenuators on the amps set?  If you're using something like the VSX26 for the processing with the subwoofer output at 0 and +24dBu maximum level outputs then you might want something like 16dB to 19dB of attenuation at the amplifier inputs.
 
Have you considered how much low frequency sound may be coming from the bass amp and drums on stage?  Might you need to control those levels in order to get the low frequency control and sound that you want?
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Ed Walters

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Re: Too much power, too many subs?
« Reply #8 on: March 31, 2014, 01:24:58 pm »

We started out with just the Crown amp and one STX828, in addition to using a PRX618S (single 18" driver, self powered) that we had from an earlier purchase. Via an Architectural Acoustics signal processor, I sent one feed to the little sub at 0dB and a separate signal to the STX828 at -6dB, since this sub combination was so lopsided power-wise.

While waiting for the second STX828 to arrive---the one that has not even been used yet---I tried just the existing STX828 without the PRX sub, and I increased the feed from -6dB to 0dB. That's when I realized I was going to have a challenge harnessing the power of two of these things! With just the one and the 6dB input increase, the LF output was massive relative to everything else.

Perhaps by selectively boosting some very low frequency from the kick or bass guitar (probably kick, I would think), I'd be able to get a powerful thump from the subs but without having to raise the higher frequencies too much. I've also thought about a sub-harmonic synthesizer. I've never used one, and I'm doubtful that such a thing is going to help. Another thing I've considered doing is using the Architectural Acoustics processor to cut everything below 50Hz. Our music doesn't have any significant content below that frequency anyway, and I believe that signals below 50Hz from the kick and bass guitar just add to the boominess. But there goes my idea of boosting very low frequencies!

Couple things come to mind.  As soon as you as say Architectural Acoustics processor, I have to wonder if you have the skills to set this up. I don't mean to be condescending but it's like those rich folks who buy exotic cars that have lots of power and promptly wreck them (e.g. Paul Cares, who totaled a Lambo LP640 five miles from where I live, racing a Toyota pickup), vs. those who buy them and learn how to drive them really fast (e.g. folks like the late Paul Newman). You can dig yourself quite the hole with a powerful DSP unit, or make a beautiful sounding rig...

So:  I am wondering if you are running proper crossover points, and the reason it's so boomy is because the crossover is too high. If it is, when you turn up the sub, you get all this mid bass mud/boom from other instruments mucking things up.  It should be 80Hz, no higher.  The high pass (cutting everything below some frequency) should be based on the limits of the speaker, and not anything like 50 Hz -- you need more like 40 Hz to get a decent kick sound anyway. The sub's manufacturer recommended high pass is 30Hz -- use that. 

Ed Walters
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Robert Weston

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Re: Too much power, too many subs?
« Reply #9 on: March 31, 2014, 08:52:07 pm »

Greetings to all! I've never posted here before, but I have a sound problem that needs solving and didn't even consider posting elsewhere. I knew I'd get good responses here!

If there's such a thing as a good problem, then that's what I've got. I have an excessive amount of low-end capability---that is, relative to the FOH volume levels that we typically run at the church where I oversee sound. In short, we have a pair of JBL PRX615M powered speakers for FOH and two JBL STX828S subs. One sub is powered by a Crown XTi4002 (3,200 watts bridged) and the other by a Crest Pro8200 (4,500 watts bridged).  For the time being, we set up each week in a school auditorium that seats around 800 or 900, and the acoustics are decent, but not great. 

I'd like to be able to feel the bass (up front near the subs, not in the back...we're not aiming to be that loud).  The only two sources that really drive the subs are kick drum and bass guitar.  I DO NOT want either the kick or electric bass to overtake the mix at the expense of everything else, since I'm looking for balanced sound. I've considered acquiring 1/3 octave EQ's or perhaps parametric EQ's so that I can boost a narrow band of specific low frequencies for the kick and bass, with the hope of creating a palpable low frequency thump while keeping the overall (or apparent) volume of the overall mix at reasonable levels.

Am I on the right track, or is there a better way to put this subwoofer power to good use? Or, do I just have a lot more bass capacity than I can use?

Thanks in advance for your suggestions.

Leonardo

Leonardo - as mentioned previously, let us know where you are.  Maybe someone on the forum is close to you and can stop by to give you some support.
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